Highly halogenated organic chemicals are favored in industry due to their many useful properties, such as stability under heat and pressure. However, these chemicals are sometimes toxic to flora and fauna. Improper disposal or spills of these organic chemicals may contaminate the environment. Cleanup is necessary due to the considerable health hazard and environmental stability of these chemicals.
In the past, an acceptable procedure for cleaning up a contaminated area involved removal of the contaminated soil or material to a designated secure landfill. Due to recent and upcoming federal regulations, the types and amounts of organic materials that can be disposed of in such a designated landfill have been greatly reduced. Therefore, a growing need exists for an efficient and economical treatment process to treat soils.
The generally accepted treatment technology for destroying highly halogenated organic contaminants is incineration. Application of incineration to soil treatment is inefficient because the contaminants to be incinerated are adhered to a large mass of inert material. Particularly in treating small quantities of soil (&lt;5000 cubic yards), incineration is inefficient because it involves collecting, packaging and transporting the contaminated material to a licensed incineration facility, heating the mass of inert solids to very high incineration temperatures to decompose the proportionately small amount of target contaminants, and packaging and returning the materials back to the site from where they were removed or disposing the materials in a secure landfill. In addition to the labor, transportation and energy costs, there is also a problem given that the capacity of presently licensed incineration facilities is limited.
A similar process to incineration for the disposal or cleanup of contaminated wastes is pyrolysis. Pyrolysis is conducted in a rotating dryer at operating temperatures on the order of 1,500.degree. to 4,000.degree. F. As with incineration, high energy costs result from the elevated temperatures.
As an alternative to incineration and pyrolysis, chemical processes were developed in order to decontaminate soil containing PCBs and chlorinated dibenzodioxins and dibenzofurans. These processes basically involve the treatment of contaminated soil with a dehalogenating agent. A typical reaction scheme involves concurrently reacting an alkali metal hydroxide with an alcohol to form an alkoxide and water; then reacting the alkoxide with the unwanted halogenated aromatic contaminant to form an ether and a salt.
In such a chemical process, the presence of water interferes with the chemical reaction scheme. Thus, the contaminated soil is preferably dried prior to the reactions. After the water has been removed, the dry, contaminated soil is treated with the reagent and the chemical reactions are carried out in a basically sealed system. To accelerate the reaction, the contaminated soil may be mixed with the reagent in an agitated vessel, possibly at an elevated temperature. The chemical treatment techniques are slow and may take weeks if not accelerated by elevated temperature, and involve the expense of spent chemicals.
Furthermore, in cases where relatively small amounts of contaminants are adsorbed to large amounts of inert materials, such as soil or sludge, each of the above techniques involves considerable expense and inconvenience. Accordingly, a keen need has been felt for a more efficient, economical system and apparatus for separating contaminants from contaminated soil, sludge and other inert materials. This need is especially evident where only small amounts (approximately 200-2,000 cubic yards) of contaminated soil or sludge need to be treated. In such situations, there is a need for a system that is adaptable to being highly transportable and cost effective.
Recently, this problem was addressed and methods to thermally decontaminate soil at temperatures below the incineration temperature were developed. Examples of these thermal treatment methods are disclosed in U.S. Pat. No. 4,997,839 (Fochtmann) and U.S. Pat. No. 4,738,206 (Noland). In these thermal processes, the contaminated soil is heated to a temperature sufficient to volatilize the contaminants which are then continuously removed from the heating chamber. Both Fochtmann and Noland disclose the fact that this process should be carried out under a slight negative pressure to avoid fugitive emissions of the volatilized contaminants. Noland also discloses that this slight negative pressure may enhance vapor stripping.
However, none of the prior art known to the applicant discloses the use of a strong or high vacuum in conjunction with a thermal decontamination process as is disclosed by the current application. The instant invention utilizes a vacuum of up to about three hundred times as strong as the negative pressure disclosed in either Noland or Fochtmann. Such a high vacuum allows the process to efficiently remove contaminants at a lower temperature or with a shorter residence time, than processes operating just below atmospheric pressure (slight vacuum). The lower operating pressure greatly reduces energy consumption flue to decreased heating needs. Vent emissions are small, thus minimizing the size of air pollution control equipment. Transport of the system to the contaminated site can therefore be accomplished relatively easily.
The present invention is also capable of treating mixed wastes. The presence of mixed wastes presents unique problems in the environmental cleanup field. Mixed wastes are contaminated both with radioactive pollutants and hazardous chemical compounds. These mixed wastes are difficult to treat or dispose. Additionally there are different legal guidelines for the handling of hazardous wastes as opposed to radioactive wastes. The Resource Conservation and Recovery Act ("RCRA") enunciates standards for the treatment, disposal and handling of hazardous wastes. However, it is the Department of Energy ("DOE") and the Nuclear Regulatory Commission ("NRC") that monitor the treatment, disposal and handling of radioactive wastes. Where a material is contaminated with both radioactive and hazardous wastes, the wastes need to be separated for further treatment or disposal.
Therefore, there is a need to develop technologies that separate the hazardous components from the radioactive components so that they can each be treated or disposed of safely and correctly. Accordingly, applicant's thermal process is useful for removing the hazardous chemical components from mixed wastes at temperatures below incineration temperatures, leaving solids that are contaminated with radioactive pollutants.
The art has recognized processes for treating mixed wastes. For instance, in U.S. Pat. No. 5,160,636 (Gilles et al.), solids contaminated with mixed wastes are treated to remove the hazardous and radioactive components from the solids. The process disclosed in this patent first extracts the contaminants from the mixed wastes solids to the liquid phase, the decontaminated solid phase is then separated from the contaminated liquid phase which contains both the hazardous and radioactive contaminants. Next the contaminated liquid stream is sent to an evaporator where the volatile contaminants are separated from the non-volatile contaminants and radionuclides for further treatment and disposal.
Additionally, processes have been practiced in the industry that use thermal decontamination to remove volatile and semi-volatile chemical contaminants from mixed wastes. These processes operate at slight negative pressures to avoid fugitive emissions.
None of the prior art known to the applicant disclose the use of the present invention is capable of removing volatile and semi-volatile chemical contaminants from mixed wastes using a thermal decontamination process at decreased pressures. The instant invention separates chemical contaminants from mixed wastes at vacuum levels up to about three hundred times as strong as known processes in the field. Such a vacuum allows the process to efficiently remove hazardous contaminants at lower temperatures or with shorter residence times, resulting in decreased energy consumption. This process removes the volatile and semi-volatile hazardous wastes in a gaseous phase and separates them for further treatment, and leaves the radioactive pollutants in the soil for proper disposal or further treatment.